Super continuum (SC) generation is a nonlinear phenomenon characterised by dramatic spectral broadening of intense light pulses passing through a nonlinear material. SC generation occurs in various media and finds use in numerous applications ranging from spectroscopy to ultra-short-pulse generation. Inter alea, optical radar and ranging (LIDAR), spectroscopy, optical computing, and reaction rate studies. Spectral slicing of the generated SC in order to replace several discrete light sources is one main mean to design such multi wavelength optical sources.
The newly developed micro structured fibres (MF) possess unique optical properties which allow generating SC with a broader bandwidth than what is possible in bulk silica or in standard optical fibres. Since the first report on SC generation in a MF in 1999 by Ranka et al. (Optics Letters, Vol. 25, No. 1, (2000), pp. 25-27) comprehensive efforts have been made to understand the physical mechanisms leading to the generation of light with a broad spectrum in this type of fibres, and an extensive literature has been published on the subject. The bulk part of these studies utilises femto-second pulses (10−15 s) to generate the SC. The physical mechanism responsible for the SC generation is believed to be the creation and fission of higher order solutions according to J. Herrmann et al., Phys. Rev. Letters, Vol. 88, No 17, 2002. It has also been shown that it is possible to create SC by use of pico- and nanosecond pulses, and the mechanism responsible for these SCs are believed to be a combination of four wave mixing and stimulated Raman scattering (Coen et al., Optics Letters, Vol. 26, (2001), pp. 1356-1358, and Town et al. Applied Physics B (Lasers and Optics), Vol. B77, No. 2-3, September 2003, pp. 235-238). The possibility of tailoring the properties of MFs for improving the efficiency of SC light generation using pico- or nanosecond pulses has, however, been little explored. The use of longer pulses is, however, attractive as it does not require an often complex and expensive femto second laser. This has so far been a main obstacle to the creation of commercially viable SC light sources.
The spectral slicing of a SC only utilizes a small part of the spectrum and there only a small part of the launched energy. The energy in the SC is often symmetrically distributed around the pump and primarily generated through a four wave mixing process or alternatively red shifted relative to the pump when stimulated Raman scattering dominates the generation process in the case of inefficient phase matching of the four wave mixing process. The hereby generated blue light will be limited to the half pump wavelength due to energy conservation of the four wave mixing process. Here the blue shifted light (idler) is generated through the action of two pump photons and a red shifted (signal) photon. The idler light wavelength generated through the four wave mixing process is determined through the conservation of energy equation: h ννdler=2 h νpump−h νsignal<=>1/λilder=2/λpump−1/λsignal, where ν and λ denote frequency and wavelength, respectively, and h is Planck's constant. For the hypothetic situation of the infrared part of the SC extending to infinity the idler wavelength minimum is to be found at the half pump wavelength.
In prior art SC from Kudlinski et al. Optics Express, Vol. 14, No. 12, 12 Jun. 2006, pp. 5715-5722 a blue extended spectrum with a flat (3 dB) spectrum from 395 nm to 850 nm, with a minimum spectral power density of 2 mW/nm (3 dBm) was achieved in a configuration with a total continuum output power of 3.5 W. The shortest wavelengths were generated by cascaded four-wave mixing: the continuous decrease of the zero dispersion wavelength along the fibre length enables the phase-matching condition to be satisfied for a wide range of wavelengths into the ultraviolet, while simultaneously increasing the nonlinear coefficient of the fibre. The manufacturing process involved in the continuous decrease of the zero dispersion wavelength along the fibre is likely difficult.
In prior art SCs shown in FIG. 1 for a 100 femto second pulse generated spectrum and in FIG. 2 for a 60 ps pulse generated spectrum there is no significant light generated below the half pump wavelength. In prior art SC from European patent application EP 1502332 by Braun and Bertram shown in FIG. 4 for a 8.5 ps 5.8 kW pulse generated spectrum light is not generated below the half pump wavelength either. This spectrum shows power intensity of the red shifted part that is equal to or smaller than the power intensity of the blue shifted part contrary to the SCs of FIG. 1 and FIG. 2. This indicates that the light is generated through another process than four wave mixing. The generated spectrum shows in the blue part a spectral density of −25 dBm/nm.
In prior art SC from Price et al. (Optics Express, Vol. 10, No. 8, Mar. 20 2002, pp. 382-387) shown in FIG. 5, a blue extended spectrum with −10 dBm/nm output power in the blue part of the spectrum for a single excitation pulse of 21 kW peak power and 350 fs duration launched into a micro-structured fibre with a 1.6 μm core diameter is shown.
Thus, there is a need for a light source providing a spectrum extending into the blue and ultraviolet part of the spectrum with a spectral density exceeding −10 dBm/nm produced in a conventional fibre design which can be manufactured on a standard drawing tower.